The disclosure relates to fuel cells and, more particularly, to PEM fuel cells and reduction in degradation of ionomer, for example in the membrane and electrodes of same.
In a PEM fuel cell, a small amount of oxygen diffuses from the cathode to the anode through the membrane and can form peroxide by reacting with hydrogen ions at low potential at the anode catalyst membrane interface. In addition, oxygen added to the fuel for carbon monoxide tolerance can form peroxide in the anode. This peroxide can dissociate into highly reactive free radicals. Free radicals may also form directly from crossover gases at catalyst surfaces within the membrane or at the electrodes. These free radicals can rapidly degrade the ionomer in the membrane and electrodes.
It is desired to achieve 40,000-70,000 hour and 5,000-10,000 hour lifetimes for stationary and transportation PEM fuel cells, respectively. Free radical degradation of the ionomer seriously interferes with efforts to reach these goals.
While numerous sources of oxygen, hydrogen and/or peroxide can contribute to this problem, hydrogen crossing over from the anode, oxygen crossing over from the cathode, oxygen in the fuel stream, and hydrogen peroxide and/or free radicals generated by the same are all issues to be addressed.
It is one object of the present disclosure to minimize degradation caused by such sources of reactive free radicals.
It is a further object of the disclosure to minimize precipitation of catalyst ions in the membrane. Such catalyst may form the sites for radical generation within the membrane.
It is a further object to provide a membrane electrode assembly having an extended lifetime due to such reduction of crossover gases, enhanced peroxide and/or radical decomposition, and reduced degradation from peroxide and/or free radicals generated.
Other objects and advantages will appear herein below.